In order to comply with tighter environmental regulations, it is required to lighten an automotive body structure further. And, in order to reduce an amount of the present material to be used, by increasing its strength, it is likely to use a high tensile steel plate for a structural member. As for the high tensile steel plate in general, the high tensile steel plate having tensile strength of 440 Mpa class or 590 Mpa class, which is easily formed by a cold press working, has been employed by now. Then, the high tensile steel plate having tensile strength of 780 Mpa class or 980 Mpa class has been used. Recently, such an ultrahigh tensile steel plate with tensile strength exceeding 1 Gpa as being called “ultrahigh-ten. steel plate” begins to be used. Furthermore, the ultrahigh tensile steel plate with tensile strength of 1.2 Gpa class or 1.5 Gpa class is being considered to be used. The higher in tensile strength the ultrahigh tensile steel plate becomes, the poorer its workability (ductility) becomes. As stress is generated when the press working was applied to the ultrahigh tensile steel plate, and released when the plate is removed from the die, such a phenomenon that the plate is elastically deformed, i.e., a so-called spring back is caused, whereby it will be difficult to ensure a desired dimensional accuracy. In addition, increase in forming load, and reduction of the die's life resulted therefrom will be concerned.
As a method for dealing with those, a hot press working (hot stamping) is generally used to form the ultrahigh tensile steel plate. This is, such a forming method that a quenching steel plate is heated up to about 900° C. in a high temperature furnace to be softened, and the press working is applied to it, and thereafter it is held at its bottom dead point to be contacted with the die, so that the quenching is strengthened by cooling effect (contact cooling). However, the hot press working is limited for such a specific material that is to be cooled uniformly even at a slow cooling speed. In addition, as it is heated at about 900° C. with its outer surface being oxidized, such a post-processing as removing oxidized scale is required. Furthermore, such specific problems are to be solved for the hot press working that not only a large exclusive heating furnace and a conveyance facility are required, but also energy consumption thereby is extremely increased. Therefore, although it is craved that the cold press working is applied to form the ultrahigh tensile steel plate, such new problems are caused that a shape of final product to be formed is limited, and so on.
For example, it is not easy to produce such final products having a U-shaped cross section or a hat-shaped cross section that are frequently used for vehicle body structural members such as vehicle members or a pillar, by means of the cold press working. As an example of the final product having the U-shaped cross section or hat-shaped cross section, it is described in Patent document 1 as listed below that in order to obtain a metallic member (press final product) having an accurate U-shaped or hat-shaped cross section, and a normal shape (designed shape) curved in a direction within a specific surface, without adjusting a die shape, and with a predetermined part shape being maintained, such a method is proposed to form a metallic member having a U-shaped or hat-shaped cross section, and having a shape curved in a longitudinal surface, that with respect to one curved part out of a plurality of curved parts formed on the metallic member, an intermediate product having a smaller radius of curvature than a shape of a final product is formed in a first forming step, and in a second forming step, it is formed into a larger radius of curvature than the radius of curvature in the first forming step, and remaining parts are formed, without changing the radius of curvature of the die in the first and second forming steps, to make the entire formed product to be formed with the product shape or substantial product shape (described in paragraphs (0004) and (0005) of Patent document 1).
Furthermore, with respect to a saddle type product to be bent in a direction opposite to an opening of a U-shaped cross section, it is described in Patent document 2 as listed below that in order to provide a drawing method without creating a crinkling on a bent portion even of a saddle type product with its gutter-shaped cross section being reversely curved, such a method is proposed to form a saddle type product curved in a reverse direction to an opening of its gutter-shaped cross section, by applying a plurality of drawing steps to a flat plate material, that in an intermediate drawing step, at least one end portion of the saddle type curved part is formed with a chipping surface of a planer surface or a curved surface, to form an intermediate shaped product having a difference of elevation between the opposite end portions of saddle type curved part, and in a finishing drawing step, it is drawn to a desired product profile, with the chipping surface being expanded to be disappeared from the intermediate shaped product (described in paragraphs (0009) and (0010) of Patent document 2).
Also, it is described in Patent document 3 as listed below that in order to provide a press die capable of obtaining a product of a high quality without crinkle, crack or the like by a single step drawing, even for a saddle type product having a large reversely curved part, and capable of expecting reduction of working time and manufacturing cost, such a method is described that by lowering an upper die, a work placed on a lower pad extruded from a lower die is clamped by an upper pad extruded from the upper die to be fixed, and by further lowering the upper die, inserting a tip end of a bead forming material into a recessed portion of a bead forming upper material, and by means of a cam driver, a cam slide, and therefore, the tip end of the bead forming material is retracted from the recessed portion to form beads, and in the next lowering step, by means of the lower punch and upper die, it is drawn into the U-shaped cross section, and in accordance with next lowering of the upper die, the restriction against the upper pad by a gas spring is relieved, so that the upper pad becomes capable of being received in the upper die, and with the beads being extended by the lower punch and upper die, the work is drawn into the final shape (described in paragraphs (0006) and (0008) of Patent document 3).
On the other hand, it is described in Patent document 4 as listed below that in order to obtain a metallic member having an accurate U-shaped or hat-shaped cross section, and a normal shape curved in a height direction, without adjusting a die shape, and with a predetermined part shape being maintained, such a multi-stage press forming method is proposed to form a metallic member having a U-shaped or hat-shaped cross section, and a shape curved in a height direction with a ceiling part surface between side walls being placed at an upper surface, that an intermediate product with an emboss protruded in the height direction in the ceiling part surface between side walls of the metallic member, so that a length of its longitudinal direction will be longer than a length of a line of a final product in its longitudinal direction, is formed in a first forming step, and in a second forming step, it is formed into a product shape with the emboss being pressed so as to be removed (described in paragraphs (0004) and (0006) of Patent document 4). Furthermore, an embodiment for forming a product with a hat-shaped cross section by use of a steel plate of 980 Mpa class with its thickness of 1.6 mm is disclosed in its paragraph (0019).